1. Field of the Invention
This invention relates to a voltage regulator circuit which, by interrupting the excitation current supplied to the field winding of a generator, regulates the output voltage of the generator to a predetermined level.
2. Description of the Related Art
Automobiles are generally provided with an AC generator the output of which is rectified by a rectifier circuit and is regulated to a predetermined voltage level by a voltage regulator circuit. In such a case, the voltage regulator circuit regulates the output voltage by interrupting the excitation current supplied to the field winding of the generator when the output voltage exceeds the predetermined level. The interruption frequency of the excitation current, however, is not constant in conventional regulator circuits, which may result in certain disadvantages.
FIG. 1 shows a voltage regulator circuit for an AC generator of an automobile. The AC generator or alternator 1 comprises an armature winding 101 and a field winding 102. A full-wave rectifier circuit 2 coupled to the output terminals of the alternator 1 is provided with a main and an auxiliary positive output terminal 201 and 202, and a grounded negative output terminal 203. The voltage regulator circuit 3 has a well-known circuit structure: A voltage divider consisting of serially connected resistors 301 and 302 is coupled across the auxiliary and the negative terminal 202 and 203 of the rectifier circuit 2, the junction between resistors 301 and 302 being coupled to the base of a control transistor 304 through a Zener diode 303. The base of the power transistor 330 coupled in series with the field winding 102 is coupled to the junction between the key switch 5 and an indicator lamp 6 through a resistor 305; the base of the power transistor 330 is also coupled to the collector of a transistor 304 having a grounded emitter. Further, a resistor 335 for supplying initial excitation current from the battery 4 to the field winding 102 is coupled in parallel with the indicator lamp 6. A diode 331 for absorbing and suppressing the surge generated by the inductance of the field winding 102 upon interruption of the field current is coupled across the field winding 102. The circuit for controlling the interruption frequency of the excitation current comprises a serial circuit of a positive feedback resistor 332 and a positive feedback capacitor 333 coupled across the collector of the power transistor 330 and the base of the control transistor 304. A biasing resistor 334 is coupled across the base of the control transistor 304 and the ground level.
The operation of the circuit of FIG. 1 is as follows. When the key switch 5 is closed to start the engine of the automobile, initial excitation current is supplied from the battery 4 to the field winding 102 through the parallel circuit of an indicator lamp 6 and a resistor 335, thereby placing the generator 1 in condition for generating power. At the same time, the indicator lamp 6 is energized to indicate that no voltage is delivered from the generator yet. When the engine is started thereafter to rotate the field winding 102, AC voltages are induced in the armature windings 101 and are rectified by the rectifier 2 into a DC voltage. When the voltage at the positive output terminals 201 and 202 rises to the level of the battery voltage, the lamp 6 is extinguished to indicate that the generation of power has begun properly. Thereafter, the regulation of output voltage is effected as follows. When the output voltage at the auxiliary terminal 202 exceeds a predetermined level to raise the voltage at the junction between resistors 301 and 302 above the Zener voltage, the Zener diode 303 becomes conductive in the reverse direction to turn on control transistor 304, thereby turning off power transistor 330. Conversely, when the output voltage at the auxiliary terminal 202 falls under the predetermined level, the Zener diode 303 regains non-conductivity to turn off control transistor 304, thereby turning on power transistor 330. The interruption frequency of the excitation current in this operation, i.e., the frequency of these repeated turning on and off of power transistor 330, is controlled by the positive feedback circuit time constant which is determined by the resistor 332 and the capacitor 333; more explicitly, the frequency is controlled by the resistance and capacitance values of the RC circuit formed by them.
The voltage regulator shown in FIG. 1, however, has the following disadvantage: Although the interruption frequency of the excitation current is controlled within a certain range by the RC circuit formed by resistor 332 and capacitor 333, it is inevitable that the frequency should vary in a wide range of from about 20 to 500 Hz due to the variation in the rotational speed of the rotor of the generator 1 and in the output voltage thereof. Thus, in cases where the interruption information of the power transistor 330 is to be fed into the microcomputer controlling the operation of the engine, the information must first be processed before being inputted, because of this wide variation of the interruption frequency. Further, if the interruption frequency becomes extremely low and falls to or even under 20 Hz, the fluctuation in the output voltage caused by the interruption of the excitation current results, for example, in a flickering of headlights of the automobile that is readily noticed by the human eye and hence is very unpleasent.